Modern dental practices, seeking economies of time at the patient's side and in the laboratory, tend to provide completed and installed implant prosthesis in as few as a single sitting. Three-dimensional images displayed and manipulated on a computer screen are derived from a CAT scan (Computer Aided Tomography) of all oral structures. Virtual implants and prosthetics are tried in this virtual space until a best case is developed. The number and type of implants, their placement angles and depths, the density of bone and the avoidance of critical structures are tested in this virtual space. Surgical drilling and implant registration guides are generated with Rapid Prototyping tools to insure an almost exact relative placement of a set of implants.
Nonetheless, minor deviations and anatomical requirements can prevent the parallel alignment of implants and the matching abutments with the final prosthesis. Under these circumstances, additional laboratory procedures such as cutting and welding to correct the undercase must be done to fit the prosthesis. One solution suggested is to provide an abutment having a smaller mating end resulting in a gap between the abutment and prosthesis for cementing, referred to as the CAL technique. In the CAL technique, a disposable shim is slipped between each abutment and substructure sleeve to make a gap to compensate for misalignment.
Izador Brajnovic in U.S. Pat. No. 7,175,434 teaches an expandable cylinder to fill the gap between the distal end of the abutment and the substructure sleeve of the undercase of the prosthesis. This is a partial solution still requiring parallel placement of abutments. Charles D. Kownacki in U.S. Pat. No. 5,302,125 offers a ball-in-socket adjustment within the upper end of the implant, leaving the distal end of abutment unmodified. This offers compensation for angular misalignment without addressing parallel displacement or vertical discrepancies of the abutments. The Kownacki placement of the ball-in-socket below the soft tissue invites bacteria and can compromise good oral hygiene.